2.0 Analysis 2.1 Brake System Components The excessive brake disc coning indicated that the discs had experienced higher-than-normal temperatures. The greater number of landing cycles accumulated by the left brake disc probably contributed to the greater coning observed on that disc. The extent of disc coning present when the brake repairs were completed could not be determined. A severely coned brake disc can cause brake drag, decreased braking efficiency and requires increased brake pedal travel to align the brake linings with the brake disc. The severely coned discs were the cause of the spongy pedal condition. The AME did not notice to what extent the left brake disc was coned when he replaced the left brake linings. However, he was able to tighten the four bolts and still rotate the tire. It is possible that the left brake was dragging and that the additional heat produced during the taxi, landing, and take-off phase that followed was responsible for the additional coning as seen on the left disc. The brake drag would have been caused by the reduced running clearance between the coned left brake disc and the new brake linings. There was no brake drag on the right brake because the right brake linings had not been replaced. It is possible that the left brake drag was great enough to produce additional heat, without a noticeable aircraft yaw to the left during ground operations. The flight crew did not experience any abnormal brake conditions before the Fox Harbour landing. The brake pedal response during an examination following the occurrence indicated that there were no internal or external leaks in the brake system. 2.2 Residual Heat and Braking Technique The brake piston insulators protect the brake cylinders from heat that is generated on the disc during aircraft braking action. The excessively coned discs, the warped brake pressure plate, and the warped and discoloured brake lining backings indicate that these components were subjected to excessive heat. When the brake disc is unable to absorb any more heat and the cylinder insulator's heat capacity is exceeded, the brake cylinder temperature can increase due to heat transfer from the brake disc and cause the brake fluid behind the cylinder pistons to boil. The heat produced when the brakes were applied during the Fox Harbour landing, combined with the residual heat from the previous landing, resulted in the brake system components becoming excessively hot. If the brake fluid was close to the boiling point when it was under pressure during brake application, then the decrease in pressure when the co-pilot released the brakes using the on-off-on braking technique could have resulted in the fluid boiling. Once the brake fluid boils, the next brake application will produce an increased brake pedal travel and decreased brake response. Had the PF maintained brake application rather than use the on-off-on braking technique, the increased brake pressure might have prevented the brake fluid from reaching the boiling point. The aircraft records indicated that 9 of the 12 brake linings were replaced on the previous day. Since the linings are capable of producing a minimum of 35 normal brake energy stops before the lining minimum thickness limits are exceeded, it can be concluded that 9 of the 12 linings would indicate minimal wear after only two landings. Since the six right brake linings exhibited much greater wear than the left brake linings, it is probable that only the six left brake linings were replaced during the brake maintenance and not the nine brake linings as identified by the Parts and Rectification sheet. It could not be determined what the right brake lining condition was when the maintenance was carried out. The left brake pressure plate loose pin condition most probably was present during the brake work as there were no impact marks around the pins to indicate that the condition was a result of the landing event. This condition could have produced a slight brake lining movement or shift when the brakes were first applied. The complete failure of both brakes, however, could not be attributed to the worn linings on the right brake or the left brake pressure plate loose pin condition. 2.4 Flight Crew Actions The captain's action of ground looping the aircraft reduced the risk of extensive damage to the aircraft and serious injury to the occupants. Had the aircraft descended the rock embankment, there would have been a risk of wing fuel tank rupture and post-crash fire. 3.0 Conclusions 3.1 Findings The aircraft was certified, equipped, and maintained in accordance with existing regulations. There was no brake system abnormality reported by the pilots prior to the Fox Harbour landing. A standard brake system was installed on the aircraft. The brake components exhibited evidence that high brake temperatures had been reached. The brake discs were coned beyond the maximum acceptable limits. Some brake linings did not meet the minimum thickness limits set out in the Component Maintenance Manual. It is probable that the brake fluid boiled during the Fox Harbour landing. The flight crew's on-off-on braking technique may not have been the most appropriate method for a short field landing. The time interval between the Mary's Harbour and Fox Harbour landings was too short to allow the brake discs to cool sufficiently. There was insufficient runway available for the flight crew to initiate a go-around after the brakes had failed. The captain's decision to ground loop the aircraft reduced the risk of extensive damage to the aircraft and serious injury to the occupants. The flight crew and passengers evacuated the aircraft without difficulty. The operator does not have a SOP manual for the PA 31-350, nor is it required to by existing legislation. 3.2 Causes Brake fluid boiling was the most probable cause of the brake failure. Contributing to this occurrence was the high energy landing in Mary's Harbour, the insufficient brake cooling time between the Mary's Harbour and Fox Harbour landings, the flight crew's braking technique, and the standard brake system installed on the aircraft. 4.0 Safety Action 4.1 Action Taken The operator has since equipped its Piper Navajo aircraft that operate into short airstrips with dual caliper heavy duty brake systems.